In transportation equipment such as automobiles and airplanes, and in industrial machinery or the like, stress is repeatedly loaded onto materials used therein by vibration and repeated driving. Therefore, it is important to grasp the fatigue life of the materials to be used, and fatigue tests are typically performed.
In conventional fatigue tests, the life is typically predicted using a load of 1×107 repeated loading. In recent years, however, there have been demands for transportation equipment and industrial machinery with extended service lives. As a result of a variety of research, for example, in metals, 1×109 repeated loading have been evaluated as a fatigue limit stress in view of the effect of metallic inclusions present in the metals.
The conventional fatigue tests are generally classified into tension-compression stress types, bending stress types, and torsional stress types. In a tension-compression stress type fatigue test which is performed with a test piece in the form of a round rod or the like, there are fatigue testing machines capable of high-speed loading by hydraulic drive, or the like, in addition to mechanical drive, and depending on the stress, high-speed tests at or above 1000 Hz can be performed. A high-speed fatigue testing device using an ultrasound oscillator has recently been developed, and such a device is suitable for giga-cycle fatigue tests. However, if a test frequency is increased in such fatigue testing devices, heat generation may occur due to strains in the test piece, and a test piece cooling system or the like is also needed.
Meanwhile, since the fatigue testing of tension-compression stress types and torsional stress types are difficult to perform with respect to sheet or plate-shaped test pieces, fatigue testing of bending stress types is typically performed. A completely reversed fatigue test in which a crank is connected to a rotating body and both ends of a test piece are mechanically bent and a fatigue test in which one end of a test piece is fixed and the end on the opposite side is mechanically vibrated are typically used to load bending stress onto a sheet or plate-shaped test piece. Therefore, since the test frequency is restricted by the rotational speed of the drive motor or the like and the heat generation preventing ability, 1800 spm is the limit, about 2 to 5 days are required to load 1×107 repeated stress, and the number of days which is larger by a factor of 100 is further required to load 1×109 repeated stress. Furthermore, a detector such as a high-precision load cell is required to detect the decrease in stress resulting from the occurrence of fatigue cracks, and so the cost of the fatigue testing device increases accordingly.
On the contrary, Patent Document 1 describes a fatigue testing device which can easily perform a giga-cycle fatigue test in a short period of time. In the fatigue test performed with this fatigue testing device, a test piece which is fixed at a proximal end side with a fixing jig is vibrated by applying a pulsed pressure wave generated from a pulse generator driven by a high-frequency motor. In this case, the vibration frequency of the test piece becomes a resonance frequency by adjusting the frequency of pressure variations in the pressure wave so as to match the resonance frequency of the test piece.
Patent Document 1: Japanese Patent Application Laid Open No. 2012-149979.